Adding sound and music to a game really transforms it. I like to leave sound till the very end. Cause once you take a huge game and add sound and music, all of a sudden it comes to life and feels really immersive. I'm not the greatest with sound effects, I usually cut them together fairly quick. So I'll share some great sound effect and music websites I use.
Worth noting is you will have to download an audio editor, some sound effects need a little chopping at the start and end or maybe make the volume louder. This will expand the amount of sound effects you can use that you find online if you are willing to edit them yourself. A great free audio editor is Audacity, I recommend downloading it.
Huge downloadable packs of sounds for free. These are more movie effects, but work very well in games. Remember with sound you can take a completely different sound effect and make it work for something else with some clever editing. The mind tricks the ear by using visual stimulation to make the sound more fitting with what they see.
Mixed feelings about this site, they used to be free and the best sound on the net, but now you have to pay. Not sure if they still offer 5 free sounds a month or got rid of it. A good option anyways if you're willing to pay for some top notch sound effects by famous sound producers.
One of my favourite, they have a lot of real sound effects if your looking for something less cartoony. Huge list of sounds as well.
Good for simple effects in a pinch or starting out. I'm not a huge fan of this resource due to the quality being a lot lower than the other links, but with sound effects sometimes you needa search multiple places looking for that right one.
My favourite place for getting music for my games. It's made by other indie developers so when you submit your game you can link the track back to there profile to show support which is a great idea for uniting sound artists together with flash artists.